MCD spectra of iron-sulfur complexes with or without inorganic sulfur

The magnetic circular dichroism spectra were observed for various iron-sulfur complexes with and without inorganic sulfur as models for rubredoxin and 2-Fe ferredoxin. The MCD band shapes ascribed the bands around 390 and 490 nm to Faraday A terms for mononuclear iron sulfur complexes. These bands are probably assigned to the charge-transfer transitions from the thiol sulfur orbital to the iron t₂ and e 3d-orbitals, respectively. The MCD magnitudes decreased by more than one-half for binuclear iron-sulfur complexes with inorganic sulfur in comparison with those for the mononuclear complexes. The low MCD magnitude as well as the possible core symmetry as low as D_2d attributed the MCD bands to Faraday B terms. Incorporation of inorganic sulfur produced new MCD bands, some of which can be assigned to the charge-transfer transitions from the inorganic sulfur orbital to the iron t₂ and e 3d-orbitals. Among complexes studied here, the bis(o-xylyldithiolato) ferrate(III) monoanion gave the MCD spectrum which resembles that of a rubredoxin. This implies that the MCD spectroscopy also assessed the complex as a good rubredoxin model. However, the binuclear complex bis[o-xylyldithiolato-μ₂-sulfidoferrate(III)] dianion failed to offer the MCD spectrum similar to that of the spinach ferredoxin.